In recent years, organic EL (organic electro luminescence (also referred to as organic LED and OLED etc.)) display has been drawing attention as a display device comprising pixels each including a light emitting element. In particular, an active matrix organic EL display in which a thin film transistor (TFT) is disposed in each pixel has actively been studied.
As the most basic pixel configuration of an organic EL display, two TFTs are used in a pixel. One of the TFTs has a function to control whether to input a video signal to the pixel or not, and referred to as a selection TFT and the like. The other TFT has a function to control the amount of current supplied to an organic EL element, and referred to as a driving TFT and the like. Depending on the level of the gate-source voltage of the driving TFT, the current value supplied to the organic EL element can be controlled.
However, the driving TFT has a problem in that its current characteristics vary among pixels, and consequently, the current value flowing in the organic EL element also varies, leading to variations in luminance.
In view of the foregoing, various pixel configurations are studied in order to prevent variations in the current value supplied to the organic EL element. As one of the configurations, a pixel to which a current but a voltage is input as a video signal to determine the luminance has been developed (see Documents 1 to 3 shown below for example). In this specification, this type of pixel is referred to as ‘a current input type pixel’. In the current input type pixel, a signal current having a corresponding value to a video signal is input to the pixel. Then, a current having a corresponding value to the signal current is supplied to an organic EL element. Since the current having the corresponding value to the signal current flows in the organic EL element even when current characteristics and the transistor size of the TFT and the like vary among pixels, variations in luminance can be prevented.
In such a current input type pixel, current is used as a video signal. Therefore, a means for supplying a current having a corresponding value to a video signal, namely a driver circuit is required. Accordingly, driver circuits for the current input type pixel are studied (see Document 4 shown below for example).
FIG. 32 shows a part of a driver circuit described in Document 4. The driver circuit shown in FIG. 32 is a circuit which inputs with a 4-bit digital voltage signal and outputs an analog current signal. That is, it has a function to convert a voltage signal into a current signal and a function to convert a digital value into an analog value at the same time. Four TFTs 3206 to 3209 are disposed as the TFTs to operate as current sources since 4-bit signals are input to the circuit.
Next, the operation of FIG. 32 is described. First, a digital voltage signal for each bit is input to input terminals 3202 to 3205. A digital voltage signal of the least significant bit is input to the input terminal 3202, and a digital voltage signal of the most significant bit is input to the input terminal 3205. Since the input terminals 3202 to 3205 are connected to the gate terminals of the TFTs 3206 to 3209 respectively, each of the TFTs 3206 to 3209 is either turned ON or OFF according to the digital voltage signal which is input to the input terminals 3202 to 3205. Then, current flows through the TFTs which are turned ON, and the total current which flows from the TFTs 3206 to 3209 is output from an output terminal 3201 according to the current rule of Kirchhoff. The current which is output from the output terminal 3201 has an analog value.
That is, the TFTs 3206 to 3209 operate as current sources, and whether a current is output or not is controlled by a digital voltage which is input from the input terminals 3202 to 3205.
The TFTs 3206 to 3209 all have the same gate length L. The gate width W differs in each TFT. The TFT 3206 has the narrowest gate width W and the TFT 3209 has the widest gate width. The gate width of each TFT is twice as large as that of the left adjacent TFT in the figure. Accordingly, when the current value which flows from the TFT 3206 as the current source for the least significant bit is referred to as 10, the current value which flows from the TFT 3207 is 2×10, the current value which flows from the TFT 3208 is 4×10, and the current value which flows from the TFT 3209 is 8×10.
Accordingly, by controlling whether or not to output a current in each of the TFTs 3206 to 3209, currents for 4-bit, namely 16 types of current value can be output from the output terminal 3201. For example, when data which turns ON the TFT is input to the input terminals 3203 and 3204, the total current of 2×10 and 4×10, namely the current value of 6×10 flows from the output terminal 3201. When data which turns ON TFT is input to the input terminals 3202, 3203 and 3204, the total current of 10, 2×10 and 4×10, namely the current value of 7×10 flows from the output terminal 3201. In this manner, an analog current signal having the corresponding value to a digital voltage signal which is input to the input terminals 3202 to 3205 is output from the output terminal 3201.
Accordingly, a 16-gray scale display is achieved when the driver circuit shown in FIG. 32 is used as a driver circuit for inputting a signal current to a current input type pixel.
FIG. 33 shows a block diagram which corresponds to the digital-analog converter circuit shown in FIG. 32. Each current source 3311 comprises an input terminal 3312 and an output terminal 3313. The input terminal 3312 is connected to the input terminal 3302 and the output terminal 3313 is connected to the output terminal 3201. The four current sources 3311 correspond to the TFTs 3206 to 3209 in FIG. 32 respectively, and each character in the rectangle denotes the current value which flows from the current source 3311. A plurality of such current sources are disposed in parallel.
The reference documents described above are cited below.
(Document 1) International publication WO01/06484
(Document 2) Published Japanese Translations of PCT International Publication for Patent Applications No. 2002-514320
(Document 3) Published Japanese Translations of PCT International Publication for Patent Applications No. 2002-517806
(Document 4) ‘Analog-Circuit Simulation of the Current-Programmed Active-Matrix Pixel Electrode Circuits Based on Poly-Si TFr for Organic Light-emitting Displays’, AM-LCD'01, p 223–226